1. Field of the Invention
The present invention relates to the oxidative stabilization of thermoplastic resins, particularly polyolefin-based thermoplastic resins.
2. Description of Related Art
Manufacturers of plastic goods have experienced a long-standing problem in stabilizing plastic resins during the high temperature, high shear, blending, and extrusion processes that are common in the industry. Resins that show excellent long-term heat stability characteristics may not be sufficiently protected against the type of degradation that occurs in an extruder during long residence times or in glass mat reinforced thermoplastic (GMT) applications where the plastic is exposed to high temperature heat fusion followed by compression molding.
GMT is a rapidly growing area in automotive under-the-hood applications. Owing to very tough processing conditions in the manufacture of GMT, the specifications for stabilizing the thermoplastic resin, as set by GMT producers, are extraordinarily high. For example, a GMT test specimen incorporating a viable antioxidant candidate must pass a long-term heat aging test for one thousand hours at 150.degree. C. In a second test, a GMT test specimen must show less than two percent weight loss after exposure at 230.degree. C. for 15 minutes. Commercially available antioxidants are capable of providing this type of stabilizing effect, if at all, only at load levels that are so high as to be economically infeasible. Thus, there is a continuing need to develop novel antioxidants, or blends thereof, that can provide the required stabilization at load levels that are lower than those currently known.
European Patent Publication Number 0 080 409 discloses rubber compounds that are useful in molding bags that resist oils at 140.degree./skbar and contain epichlorohydrin polymers (100 parts), plasticizers (0.5 to 20 parts), reinforcing filters (1 to 150 parts), metal stearates (0.5 to 7.5 parts), Pb.sub.3 O.sub.4 (0.5 to 7.5 parts), stabilizers (0.5 to 5.5 parts), and vulcanizing agents (0.5 to 5.5 parts). Thus, a mixture of 100 parts polyepichlorohydrin, 4 parts stearic acid, 2.5 parts Pb.sub.3 O.sub.4, 1 part zinc stearate, 2.5 parts N-isopropyl-N'-phenyl-p-phenylenediamine, 1 part (Bu.sub.2 NCS.sub.2).sub.2 Ni, 0.75 part H.sub.2 N(CH.sub.2).sub.6 COOH, and 30 parts carbon black was vulcanized 40 minutes at 160.degree. to give a rubber having a tensile strength of 18.7 MPa, elongation 900 percent, 300 percent modulus 6 MPa, and Shore hardness 61; compared with 16.3, 720, 7.6, and 64, respectively, after 24 hours in air at 140.degree.; and 14.3, 500, 9, and 60, respectively, after 24 hours in oil at 140.degree..
U.S. Pat. No. 4,797,511 discloses a composition stabilized against oxidative degradation comprising: a polyolefin, carbon black homogeneously incorporated in said polyolefin, a stabilizing amount of thiodiethylene bis(3,5-di-t-butyl-4-hydroxy)hydrocinnamate (Naugard 35 or EL50) first stabilizer component incorporated into said polyolefin and a second stabilizer component of at least one amine antioxidant selected from the group consisting of a para-substituted aralkyl-substituted diphenylamine; a para-phenylenediamine and a polymerized dihydroquinoline incorporated into said polyolefin.
U.S. Pat. No. 4,837,259 discloses polypropylene stabilized against oxidative degradation by the presence therein of a stabilizing amount of an antioxidant composition that comprises:
(a) at least one aralkyl-substituted diarylamine; and PA1 (b) at least one sterically hindered phenol. PA1 (A) a thermoplastic resin; and PA1 (B) a stabilizing amount of a blend of at least one sterically hindered phenol antioxidant, at least one secondary amine antioxidant and/or at least one N,N'-substituted oxamide, and at least one thioether antioxidant. PA1 (A) a thermoplastic resin; PA1 (B) up to about 10 percent by weight carbon black; PA1 (C) up to about 50 percent by weight glass; and PA1 (D) a stabilizing amount of: PA1 (A) a first stabilizer comprising at least one N,N'-substituted oxamide antioxidant and/or at least one secondary amine antioxidant selected from the group consisting of: PA1 (B) a second stabilizer comprising at least one sterically hindered phenol antioxidant; and PA1 (C) a third stabilizer comprising at least one thioether antioxidant. PA1 Antagonism between antioxidants would be expected if interactions occur which would destroy or reduce the effectiveness of either component. Hawkins and coworkers . . . observed an antagonistic effect when carbon black, used to protect against outdoor weathering, was combined with secondary amines or certain hindered phenols . . . . PA1 Carbon black is a weak thermal antioxidant for polyethylene at 140.degree. C., inhibiting oxidation for only about 20 h when used alone. The amine, on the other hand, provides effective protection for over 450 h. When the two are used in combination, however, the polymer is protected for only about 100 h. Carbon black also reacts antagonistically with many hindered phenols. PA1 Absorption of amines or phenols onto the carbon black surface has been suggested as an explanation for the antagonistic effect. However, the extent of the antagonism varies with the chemical structure of carbon blacks, suggesting that catalytic destruction of the antioxidant at the surface may be the major factor responsible for antagonism. There are certain phenols which exhibit synergism with carbon black rather than antagonism. PA1 R.sub.1 is selected from the group consisting of phenyl and p-tolyl radicals; PA1 R.sub.2 and R.sub.3 are independently selected from the group consisting of methyl, phenyl, and p-tolyl radicals; PA1 R.sub.4 is selected from the group consisting of methyl, phenyl, p-tolyl, and neopentyl radicals; PA1 R.sub.5 is selected from the group consisting of methyl, phenyl, p-tolyl, and 2-phenylisobutyl radicals; and, PA1 R.sub.6 is a methyl radical. ##STR2## PA1 R.sub.1 through R.sub.5 are independently selected from the radicals shown in Formula I and R.sub.7 is selected from the group consisting of methyl, phenyl, and p-tolyl radicals; PA1 X is a radical selected from the group consisting of methyl, ethyl, C.sub.3 -C.sub.10 sec-alkyl, .alpha.,.alpha.-dimethylbenzyl, .alpha.-methylbenzyl, chlorine, bromine, carboxyl, and metal salts of the carboxylic acids where the metal is selected from the group consisting of zinc, cadmium, nickel, lead, tin, magnesium, and copper; and, PA1 Y is a radical selected from the group consisting of hydrogen, methyl, ethyl, C.sub.3 -C.sub.10 sec-alkyl, chlorine, and bromine. ##STR3## PA1 R.sub.1 is selected from the group consisting of phenyl or p-tolyl radicals; PA1 R.sub.2 and R.sub.3 are independently selected from the group consisting of methyl, phenyl, and p-tolyl radicals; PA1 R.sub.4 is a radical selected from the group consisting of hydrogen, C.sub.3 -C.sub.10 primary, secondary, and tertiary alkyl, and C.sub.3 -C.sub.10 alkoxyl, which may be straight chain or branched; and PA1 X and Y are radicals selected from the group consisting of hydrogen, methyl, ethyl, C.sub.3 -C.sub.10 sec-alkyl, chlorine, and bromine. ##STR4## PA1 R.sub.9 is selected from the group consisting of phenyl and p-tolyl radicals; PA1 R.sub.10 is a radical selected from the group consisting of methyl, phenyl, p-tolyl and 2-phenyl isobutyl; and PA1 R.sub.11 is a radial selected from the group consisting of methyl, phenyl, and p-tolyl. ##STR5## PA1 R.sub.12 is selected from the group consisting of phenyl or p-tolyl radicals; PA1 R.sub.13 is selected from the group consisting of methyl, phenyl, and p-tolyl radicals; PA1 R.sub.14 is selected from the group consisting of methyl, phenyl, p-tolyl, and 2-phenylisobutyl radicals; and PA1 R.sub.15 is selected from the group consisting of hydrogen, .alpha.,.alpha.-dimethylbenzyl, .alpha.-methylbenzhydryl, triphenylmethyl, and .alpha.,.alpha. p-trimethylbenzyl radials. PA1 R.sub.9 is phenyl and R.sub.10 and R.sub.11 are methyl.
The stabilizing effect of these blends was demonstrated by measurement of the retention of melt-flow rate and Hunter color value. However, while the patentees clearly demonstrated the stabilizing effect of the blends by measuring retention of melt-flow rate and color hold of stabilized resin, stabilization as evidenced by oven aging testing, the most critical test method used in the GMT industry, was not disclosed. The present inventors tested blends within the scope of this patent for oven aging stability and found that a polypropylene film stabilized with a given blend of a diarylamine and a sterically hindered phenol gave only 200 hours at 150.degree. C., far short of the GMT requirement for 1,000 hours.
European Patent Publication Number 0 328 788 discloses a composition comprising 100 parts by weight (pbw) of an aromatic vinyl compound-conjugated diene block copolymer and 1.0 to 5.0 pbw of a mixture of two or more compounds chosen from a dithiocarbamate derivative, a triazine derivative and a polyphenol compound, an adhesive composition containing it, and a process for the preparation of said compositions by mixing the components in question.
JP 71037440 discloses the use of thiourea, Irganox RA-565, Soxinol PZ {(Me.sub.2 NCS.sub.2).sub.2 Zn}, Irganox RA-1093, and Soxinol M (2-mercaptobenzothiazole) as discoloration stabilizers for polypropylene fibers and films containing a phenolic antioxidant. For example, a polypropylene textile containing 0.25 percent Irganox RA-1010 and impregnated with a 5 g/l thiourea solution (wet pickup 80 percent) was not discolored by ammonia, while an unfinished textile became pink.
Hwahak Kwa Hwahak Kongop (1975), 18(6), 302-9 discloses the use of tetraethylthiuram disulfide (I) and tetramethylthiuram disulfide (II) as heat stabilizers for S-modified neoprene rubber. The scorch time linearly increased with increasing I and II content, while Ni bis(N,N-dibutyl dithiocarbamate) (III), N-phenyl-2-naphthylamine (IV), thiophenylamine (V) and BHT accelerated the crosslinking. III, IV, and V were good antioxidants and prevented dehydrochlorination. The rubber containing 1 percent I had a storability (plasticity decrease by 70 percent) of 1.5 years.
WO 95/25074 discloses an aqueous chemical treatment or size for silaceous fibers and fillers that has a polyolefin compatible film-forming polymer, an organo-functional coupling agent, and at least one stabilizer to produce a polyolefin reinforcing article. The stabilizer is one or more of the following: alkali metal and alkaline earth metal and ammonium, phosphinates, phosphites, hypophosphites, sulfites and bisulfites, organic phosphinates and/or phosphites and mixtures thereof and in combination with other types of antioxidants like hindered phenols, diarylamines, thioethers, wherein the amount of stabilizer is in an effective stabilizing amount. Optionally the size can have one or more partial esters of a branched carboxylic acid copolymer present.
U.S. Pat. No. 5,646,207 discloses an aqueous sizing composition for glass fibers said to be particularly useful for the reinforcement of thermoplastic or thermosetting matrix polymers that contains a film-forming material, fluorescent whitening agent, coupling agent, stabilizing agent and lubricant. The film-forming material is selected to be compatible with thermosetting or thermoplastic matrix polymer used in the ultimate forming process. The fluorescent whitening agent is at least dispersed, emulsified or solubilized in water.
The sizing compositions are said to be particularly advantageous for sizing glass fibers to be used in the reinforcement of matrix polymers for forming molded parts. The resultant molded part exhibits exceptional whiteness and comparable physical properties to materials without the brightening agent.
Gachter/Muller: Plastics Additives Handbook, Hanser Publishers, 4.sup.th ed., describes the antioxidant activity of sterically hindered phenols, diarylamines, and thioethers, albeit not in the context of ternary blends. See especially pages 40, 41, 44-47, and 52-55. It is stated on page 54, "The most important long-term heat stabilizers for polypropylene are phenols of medium (300-600) and especially high (600 to 1200) molecular weight. They are frequency used together with thioethers as synergists, e.g. dilauryl thiodipropionate . . . , or distearyl thiodipropionate . . . , or dioctadecyl disulfide . . . ."
The disclosures of the foregoing are incorporated herein by reference in their entirety.